Continuously operable vertical digester for cooking pre-impregnated fibrous raw materials



Aug. 11, 1964 1.. SIMON 3,144,381

CONTINUOUSLY OPERABLE VERTICAL DIGESTER FOR COOKING PRE-IMPREGNATEDFIBROUS RAW MATERIALS Filed July 11, 1961 2 Sheets-Sheetv 1 FIG. 7

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CONTINUOUSLY OPERABLE VERTICAL DIGESTER FOR cooxmc PRE-IMPREGNATEDFIBROUS RAW MATERIALS Filed July 11, 1961 2 Sheets-Sheet 2 I 1 E g lFIGS 11 5:21-51? 6 i:

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INVENTOR L up WIG S/MON y ATTORNEYS United States Patent 3,144,381CONTHJUOUSLY OPERABLE VERTICAL DI- GESTER FOR COOKING PRE-IMPREG- NATEDFIBROUS RAW MATERIALS Ludwig Simon, St. Polten, Austria, assignor to J.M.

Voith Aktiengesellschaft, St. Polten, Austria, a corporation of AustriaFiled July 11, 1961, Ser. No. 123,162 Claims priority, applicationAustria July 20, 1960 5 Claims. (Cl. 162-237) This invention relates toa continuously operable vertical digester for cooking pro-impregnatedfibrous raw materials for the manufacture of chemical or semi-chemicalpulp at temperatures over 100 C. and under pressure.

It is an object of the invention to provide a digester which is suitablefor a continuous cooking operation in the vapor phase.

It is desired in industry to increase the productivity of labor and tosave raw materials. In long years of research, the semi-chemical processhas been developed in the pulp industry in order to reduce the rawmaterials required in the manufacture of pulp and to reduce also theprice of the various products. The next step was the continuousdigestion by cooking in the vapor phase, using pre-impregnated rawmaterial. This resulted in a considerable reduction in steam consumptioncompared to cooking in a liquid phase and in a reduction of labor costscompared to the discontinuous operation. In addition, the cookingtemperature was increased above the usual temperature to enable a betterutilization of a given digester volume by a reduction of the cookingtime. For the same reason, the surplus of digester chemicals has beenused with good results.

There is an additional measure which can be adopted to reduce the costsof the manufacture of pulp from preimpregnated raw materials. Thisconsists in the use of digesters having a large Volume.

Several proposals have already been made to achieve this aim. Mostdigesters of the type mentioned initially hereinbefore are providedadjacent to the bottom with stirrer arms, scrapers, spiral scraperblades or the like for loosening the material and subsequently to effector facilitate its discharge. A considerable portion of the pulp columnrests on these means so that they are under a high pressure load as theyrotate together with parts of the pulp column. Some of the digestersdesigned according to known proposals are hardly suitable for thisfunction. The relatively short shafts used to rotate said means areunder compressive stress and it is not possible owing to the hightemperature to provide bearings within the digester or the dischargeregion thereof in order to reduce the buckling stress acting on theshaft.

Vertical digesters have been disclosed which comprise a shaft extendingthroughout the height of the digester. In addition to otherdisadvantages inherent in part of these digesters, their maindisadvantage resides in the provision of shaft bearings inside thedigester. The function and life of these hearings are adversely affectedby the existing temperature conditions.

All these difficulties have been eliminated by the invention. As hasbeen mentioned initially hereinbefore, the invention relates to acontinuously operable vertical digester for cooking pro-impregnatedfibrous raw materials for the manufacture of chemical or semi-chemicalwoodpulp at temperatures above 100 C. and under pressure, which digestercomprises a centrically mounted, driven vertical shaft, a digesterbottom having one or more discharge openings which are suitablyconnected to one or more cylindrical or conical screw-type extruders orthe like and/ or a disintegrator, which digester is characterized inthat the shaft extends throughout the digester vessel Patented Aug; 11,1964 from top to bottom and is mounted in known manner in an axialbearing on the cover of the vessel and guided in a radial bearing belowthe bottom of the vessel, and that a discharge device carrying theentire pulp column is supported by the shaft next to the digesterbottom. In a preferred embodiment, the diameter of the shaft may belarger at the top end than at the bottom end of the digester. The shaftmay be hollow and formed with holes, through which the steam introducedinto the shaft enters the interior of the digester.

Further advantages and details of the invention will be explained withreference to the accompanying drawings, in which FIG. 1 is alongitudinal sectional view showing one embodiment of a digesteraccording to the invention.

FIG. 2 is a sectional view taken on line ab of FIG. 1.

FIGS. 3, 4 and 5 are diagrammatic views showing modifications.

The drawings and the following specification disclose also features thatare not a part of the invention but are required to understand the sameand in any case suitable.

The vertical cylindrical digester 1 operates continuously in the vaporphase and serves for cooking pre-impregnated fibrous raw materials formanufacturing chemical or semi-chemical pulp at temperatures over C. andunder pressure. The digester may also be used for pretreating anyfibrous raw material.

The material to be cooked is introduced into the digester 1 from ahopper 35. The Weight of the material 3 to be cooked in the digesterloads the rotating outlet device 4, which is secured to a continuous,vertical shaft 5 having a controllable speed. The shaft 5 is supportedby an axial bearing (step bearing) 6 carried on the cover of thedigester 1. A lower bearing 7 also disposed outside the digester servesas a guide bearing for the shaft. Owing to this arrangement thecontinuous vertical shaft 5 is stressed only in tension and torsionrather than in fiexion because the load of the discharge device 4 isuniformly divided by the material 3 to be boiled. For this reason thevolume of the digester may be large without involving structuraldifficulties because the digester having a given height may be given anappropriate diameter.

As is known per se, the material 3 to be boiled must slowly descend inthe digester 1. This descent must be uniform in order to ensure auniform cooking. The discharge device 4 must continuously and uniformlyremove the cooked material from the digester. The chips of raw materialdescend in the digest 1 under the action of the force of gravity, whichcan act on the material 3 being cooked without hindrance in allcross-sections unless the chips form bridges. To fulfil thisrequirement, the available cross-section of the digest 1 must bedownwardly enlarged in known manner to provide for a progressive reliefof the chips of raw material.

In the embodiment shown in FIGURE 1 this results from the shape of thecontinuous vertical shaft 5, the the diameter of which at the top end ofthe digester 1 is larger than at the lower end of the digester at theoutlet device 4.

The digester is provided with a device 8 of any known type forindicating the level of the material to be cooked. It is also suitableto provide the continuous vertical shaft 5 at its top end with stirringarms 9 for leveling the material 3 to be cooked.

Steam at more than 100-200 C. is introduced through the steam supplyconduit 10 into the digester 1 to heat the material to be cooked. Asteam pipe 11 is connected to the digester approximately in the middleand also below the outlet device 4, e.g. by means of a ring conduitprovided with a plurality of supply connections, in order to equalizethe steam pressure in the digester. An

automatic device 12 is provided to eliminate air and gasses from thedigester 1.

If, in accordance with the invention, the shaft is hollow and providedwith holes, steam can be supplied also from the top by means of theconduit indicated with dash-and-dot lines.

The rotating outlet device 4 driven by the shaft 5 at, e.g. 20-30revolutions per minute, consists of a disc 13 having approximately thesame diameter as the bottom of the digester, and two or more stirringarms 14 which are disposed below the disc 13 and curved opposite to thedirection of rotation 15. The stirring arms 14 are rigidly secured tothe underside of the disc 13. Pegs or pins 16 in the form of pyramids,cones, prisms, cylinders or the like are secured to the top face of thedisc 13. The disc 13 is also formed with through holes 17. During therotation of the outlet device 4, the pins mix the cooked chips of theraw material.

At the same time, these chips are comminuted to some extent at thecooking temperature because the weight of the material 3 being cookedpresses the cooked material onto the disc 13. Owing to the frictionbetween the chips and between the chips and the digester wall thematerial being cooked cannot rotate in synchronism with the disc 13.Owing to the pressure exerted by the material 3 being cooked on the topface of the disc 13 and owing to the force of gravity, the cooked andcomminuted raw material falls through the holes 17 into the spacebetween the disc and the digester bottom, in which space the stirrerarms 14 rotate in the direction 15. They are preferably curved oppositeto the direction of rotation. Owing to the shape of the stirrer arms 14,the material is urged outwardly towards the discharge opening 19 in thedirection of the arrow 18 and flows through a discharge conduit 20 intoa conical or cylindrical screw-type extruder 21, which is radially ortangentially disposed.

The rotating screw 22, which is driven, e.g. by an electric motor havinga controllable speed, compresses the material and feeds it to theadjustable discharge opening 23, through which it is extruded from theextruder 21. The material is blown out in the direction of the arrow 24by means of steam.

The shaft 5 is connected by a clutch 27 to a stub shaft of thetransmission 28, which is driven by an additional electric motor.

FIG. 3 shows another design of the digester providing for a relief ofthe chips of the raw material in the digester. To this end, the digester1a is assembled from a plurality of relatively short cylindrical parts.The diameter of each part is larger than that of the part preceding inthe direction of flow. This digester provides for a stepwise enlargementof the free cross-section of the digester.

FIG. 4 shows another form of the digester 1b, which has the shape of anupright cone providing for a continuous increase of the freecross-section of the digester from top to bottom to afford a progressiverelief of the chips of the raw material.

FIG. 5 shows a digester in which features of the digesters of FIGS. 3and 4 are combined. This digester 10 consists of a plurality of conicalparts and the top diameter of each part is larger than the bottomdiameter of the part preceding in the direction of flow. This providesfor an increase of the free cross-section of the digester in thedownward direction and for a stepwise and continuous relief of the rawmaterial.

According to the invention the digester may have a conical shaft 5(FIG. 1) or a cylindrical shaft 5a (FIG. 3), 5b (FIG. 4) or 5c (FIG. 5).This conical or cylindrical shaft is desirably provided with a pluralityof projections, pins or the like, which extend substantially at rightangles to the shaft and prevent a formation of bridges by the materialbeing cooked.

The invention provides an arrangement of the shaft and of the dischargedevice carried thereby in which the shaft is stressed only in tensionand possibly in torsion and the bearings disposed outside the digesterare exposed only to the environmental temperature and are easilyaccessible for maintenance. As contrasted with known designs, thefeatures of the invention enable the construction of reliable digestershaving a large volume without involving design difficulties.

V/hat is claimed is:

1. A vertical digester for the continuous cooking of preimpregnatedfibrous raw materials in the manufacture of pulp at temperatures aboveC. and under pressure, comprising a digester vessel adapted to contain apulp column and having a top and a bottom, said bottom having at leastone outlet, a vertical shaft coaxially disposed in and extendingthroughout said digester vessel and through the top and bottom thereof,an axial thrust bearing for said shaft located above said top, a radialbearing for said shaft disposed below said bottom, drive means forrotating said shaft in said axial and radial bearings, a dischargedevice fixed to and carried by said shaft above and adjacent to saidbottom, said discharge device comprising a disc secured to said shafthaving perforations as well as projections provided on its top face andhaving approximately the same diameter as the digester bottom, and atleast two stirrer arms rigidly secured to said disc on the undersidethereof and serving for pushing the digested chips collected at thebottom of the vessel radially outwards.

2. A digester as set forth in claim 1 wherein the diameter of said shaftis larger at the top than at the bottom of said digester vessel.

3. A digester as set forth in claim 1 wherein said shaft is hollow andprovided with holes so that steam can pass through said holes into theinterior of said digester vessel.

4. A vertical digester for the continuous cooking of preimpregnatedfibrous raw materials in the manufacture of pulp at temperatures above100 C. and under pressure, comprising a digester vessel adapted tocontain a pulp column and having a top and a bottom, said bottom havingat least one outlet, a vertical shaft coaxially disposed in andextending throughout said digester vessel and through the top and bottomthereof, an axial thrust bearing for said shaft located above said top,a radial bearing for said shaft disposed below said bottom, drive meansfor rotating said shaft in said axial and radial bearings, a dischargedevice fixed to and carried by said shaft above and adjacent to saidbottom and the diameter of said shaft being larger at the top than atthe bottom of said digester vessel.

5. A vertical digester for the continuous cooking of preimpregnatedfibrous raw materials in the manufacture of pulp at temperatures above100 C. and under pressure, comprising a digester vessel adapted tocontain a pulp column and having a top and a bottom, said bottom havingat least one outlet, a vertical shaft coaxially disposed in andextending throughout said digester vessel and through the top and bottomthereof, an axial thrust bearing for said shaft located above said top,a radial bearing for said shaft disposed below said bottom, drive meansfor rotating said shaft in said axial and radial bearings, a dischargedevice fixed to and carried by said shaft above and adjacent to saidbottom, said shaft being hollow and provided with holes so that steamcan pass through said holes into the interior of said digester vessel.

References Cited in the file of this patent UNITED STATES PATENTS1,415,433 Engel May 9, 1922 2,027,131 Upton Jan. 7, 1936 2,516,447Burling July 25, 1950 2,540,706 Beck Feb. 6, 1951 2,858,211 Durant Oct.28, 1958 2,960,161 Richter Nov. 15, 1960

5. A VERTICAL DIGESTER FOR THE CONTINUOUS COOKING OF PREIMPREGNATEDFIBROUS RAW MATERIALS IN THE MANUFACTURE OF PULP AT TEMPERATURES ABOVE100*C. AND UNDER PRESSURE, COMPRISING A DIGESTER VESSEL ADAPTED TOCONTAIN A PULP COLUMN AND HAVING A TOP AND A BOTTOM, SAID BOTTOM HAVINGAT LEAST ONE OUTLET, A VERTICAL SHAFT COAXIALLY DISPOSED IN ANDEXTENDING THROUGHOUT SAID DIGESTER VESEL AND THROUGH THE TOP AND BOTTOMTHEREOF, AND AXIAL THRUST BEARING FOR SAID SHAFT LOCATED ABOVE SAID TOP,A RADIAL BEARING FOR SAID SHAFT DISPOSED BELOW SAID BOTTOM DRIVE MEANSFOR ROTATING SAID SHAFT IN SAID AXAIL AND RADIAL BEARINGS, A DISCHARGEDEVICE FIXED TO AND CARRIED BY SAID SHAFT ABOVE AND ADJACENT TO SIADBOTTOM, SAID SHAFT BEING HOLLOW AND PROVIDED WITH HOLES SO THAT STEAMCAN PASS THROUGH SAID HOLES INTO THE INTERIOR OF SAID DIGESTER VESSEL.